Anisotropic Conductive Film (ACF) for Fine Pitch Bonding

• 1. What is anisotropic conductive film (ACF)?
• 2. How ACFs can bond, conduct, and insulate
• 3. The ACF structure and bonding process
• 4. The advantages of bonding components with ACFs
• 5. The history of ACF and its innovation
• 6. Selecting the best type of ACF

What is anisotropic conductive film (ACF)?

Anisotropic Conductive Film (ACF) serves as an adhesive for securing and electrically connecting electronic components, like integrated circuits (ICs), to circuit boards.

Originally introduced in 1977, ACF is now a staple in nearly all digital devices that employ flat panel displays—think smartphones, tablets, and high-definition TVs.

ACFs are typically used to connect the display panel to the flexible substrate that transmits signals to the panel or to the IC. Common applications of ACF include: COG (Chip-on-Glass), which connects the IC and glass substrate FOG (Flex-on-Glass), which connects the flexible substrate and glass substrate FOB (Flex-on-Board), which connects the flexible substrate and rigid substrate Other applications of ACF extend to smart cards, camera modules for CCDs (charge-coupled devices), and CMOSs (complementary metal oxide semiconductors). Shenzhen olian design and manufactur all the COG,COF,COP,FOG,FOB,FOP,FOF,TFOG,TFOF ACF bonding machines.

How ACFs can bond, conduct, and insulate

Anisotropic Conductive Film serves three primary functions simultaneously: adhesion, electrical conduction, and insulation. This unique capability allows manufacturers to connect a large number of pads at once.

Unlike traditional soldering, ACF enables fine pitch connections. The film is composed of conductive particles dispersed in a thermoset resin. A typical particle features a polymer core coated with nickel or gold, covered by an insulating layer.

When heat and pressure are applied, the opposing pads capture these particles. This process breaks the insulating coating to establish a vertical electrical connection. Particles trapped between pads remain insulated, preventing short circuits.

Key Technical Specifications:

• Bonding Temperature: Ranges from 110°C to 180°C.
• Film Thickness: Available from 10 to 45μm.
• Film Width: Ranges from 0.5 to 20mm.
• Roll Length: Available from 10 to 300m.

The ACF structure and bonding process

The bonding process is precise and efficient. Shenzhen Olian recommends following these steps for optimal results:

1. Clean the surface of the board to be attached.
2. Apply heat and pressure to the ACF with the release liner attached.
3. Peel off the release film carefully.
4. Align the pads of the IC chip to the ACF.
5. Apply heat and pressure again to finalize the bond.

ACFs are sold in reels, as shown in the photo below. They come in a wide range of film thicknesses. From 10 to 45μm, widths from 0.5 to 20mm, and lengths from 10 to 300m.

The figure below illustrates two types of ACFs: a three-layer type, where ACF is sandwiched between a protective film and a release film.

And a two-layer type where only the release film is used. Each type offers different benefits. The three-layer type reduces the risk of dust contamination, while the two-layer type eliminates the need to remove the protective film during bonding.

The advantages of bonding components with ACFs

As technology advances each year, circuit boards are becoming increasingly fine pitch, the area of connecting pads is shrinking, and the spacing between them is narrowing.

While reflow soldering and connector components are traditionally used for bonding electronic components such as ICs to circuit boards, ACFs provide a solution to these trends.

However, a disadvantage of ACF is that it is challenging to bond components of different shapes simultaneously, which is common in reflow soldering.

Also, since ACF is an adhesive, it cannot be freely removed and reattached like mechanical connectors.​​

Why Choose ACF?

Benefits of ACF

• Can bond components on glass substrates
• Enables batch connections of multiple pads
• Enables fine pitch connections
• Pb-free
• Quick bonding at relatively low temperatures
• Thinner bonding area

The history of ACF and its innovation

Dexerials has been continually enhancing ACF since its introduction in 1977. Initially, the conductive particles used were carbon fiber and solder particles. However, in 1988, Dexerials developed particles plated with nickel and gold.

By the 1990s, Dexerials had successfully developed a technology to coat the surface with insulating material.

As digital devices have evolved towards higher definition, Dexerials has adapted to the trend of fine pitch connections by reducing the particle size from 5μm to 2.8μm.

In 2014, Dexerials developed a technology to uniformly align conductive particles in thermosetting resin.

And in 2016, began marketing the product as “particle-arrayed Anisotropic Conductive Film” (ArrayFIX). This product contributes to the miniaturization, slimming design of digital devices, and higher resolution displays.​​

Selecting the best type of ACF

When selecting the best ACF, several factors should be considered, including the type of adherend, connection area, distance between pads, height of pads, and heat resistance.

ACF is composed of either epoxy resin or acrylic resin that cures with heat. we recommends acrylic ACF for applications where Flexible Printed Circuits (FPCs) and substrates (PCBs or glass substrates) are to be bonded, and where the design is intended to be reworked after the main bonding. (Please note that Dexerials does not recommend reworking itself. Dexerials advises that rework should only be done after reliability evaluations have been conducted to ensure that there are no issues.) Dexerials does not recommend ACF with epoxy resin if there is a possibility that the customer will rework. The primary reason is that it often damages the substrate to be reworked. This is because cured epoxy resin does not dissolve in solvents, and the process involves scraping off the residue.

Reworking after bonding typically involves heating the bonding area with a heat gun, hot iron, hot plate, etc., and then peeling off the FPC. Afterward, the cured resin residue between the pads on the board should be carefully removed using a solvent such as Methyl Ethyl Ketone (MEK) or N-Methyl-2-pyrrolidone (NMP) and a cotton swab. However, Dexerials does not recommend reusing FPCs because FPCs often curl during the peeling process and cleaning the resin residue between the pads on the FPC side is challenging.

With over 40 years of leadership in ACF technology, Dexerials can provide services including the initial setup, proposing an appropriate ACF, and post-connection analysis and evaluation. If you have any questions about ACF application, please feel free to contact us.

About Shenzhen Olian

Shenzhen Olian Automatic Equipment Co., Ltd ,established in 2012, is a national high-tech enterprise specializing in the research and development, sales, production, and service of automation equipment for FPD flat panel displays and new flexible screen displays.  The company holds multiple invention patents, utility model patents, and software copyrights.  At present, the company has a professional R&D team of more than 100 workers, as well as a professional team for production management, quality management, and after-sales service.

Olian’s products include LCM module factories and full process equipment (COG COF COP ,FOG FOB FOF FOP FOC TFOG TFOF bonding, AOI, dispensing, backlighting, soldering), achieving manufacturing of integrated display and touch products.  The company’s series of products are widely used in display fields such as mobile phones, electronic tags, tablets, smart wearables, automotive displays, televisions, computers, etc., and provide overall solutions for smart factories.

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